KR20010025306A - Symmetrical two hybrid lenses - Google Patents

Abstract

PURPOSE: A diffraction/refraction composite lens having a symmetrical face based on an iris is able to design lateral color, coma and distortion color that are minimized by mutual cancellation effect. CONSTITUTION: A composite image-formation lens using a diffraction plane and a refraction plane is composed of two lenses having a symmetrical plane. The lenses are identical to each other. The lenses are arranged in order of convex, concave, concave and convex or in order of convex, convex, convex and convex.

Description

Symmetrical two hybrid lenses with mutually symmetric planes based on aperture

The present invention is composed of a total of two identical lenses having mutually symmetrical planes based on the aperture, and thus the lateral chromatic aberration (or magnification chromatic aberration), coma, and distortion aberration are structurally small. Such a structure produces almost the same amount of surface deterioration based on the aperture even if surface deterioration (particularly, a relief structure of the diffractive surface) occurs when manufactured by plastic or glass molding, so the above-described aberrations are caused by mutual offsetting effects. Minimum.

In addition, the feature of the present invention is that by forming the same lens on the basis of the aperture, the manufacturing cost can be lowered by the molding method, and aberrations including chromatic aberration can be corrected at high speed and wide angle due to simultaneous use of the diffractive surface and the refractive surface. can do. In addition, compared to a diffractive optical element composed of a single lens, the focal length is reduced, which has the advantage of making it compact. In addition, when the diffraction surface is arranged on the diaphragm side, the maximum number of zones (diffraction rings) can be reduced, thereby minimizing the decrease in optical performance due to the diffractive optical element.

The following three things can be mentioned for the objective of this invention.

1) Design an image forming lens system with structurally small lateral color or magnification chromatic aberration, coma, and distortion aberration.

2) Design a lens system for image imaging with structurally small aberration caused by deterioration of surface accuracy (especially relief structure of diffractive surface) by mass production of plastic and glass.

3) Design a wide angle of view, high speed, low cost, simple configuration, thin and small image imaging lens system.

Lenses and prisms of ordinary optical elements use the refraction and reflection of light. Recently, however, interest in diffractive optical devices that control the phase or amplitude of the wavefront through changes in refractive index, height, and period of surface relief has increased.

The diffraction element basically satisfies the following lattice equation.

Where n 'is the refractive index on the exit surface side, n is the refractive index on the entrance surface side, θ' is the exit angle, θ is the incident angle, λ is the wavelength of the light used, m is the diffraction order, P is the pitch of the diffraction grating. Determine the shape.

By properly designing the pitch of the diffraction grating using equation (a), the diffraction grating acts as a lens.

The radius of the k (integer) zone can be defined by the following equation.

Diffraction elements can be broadly divided into amplitude type elements and phase type elements. In the case of amplitude type elements, the diffraction efficiency is about 34%, and in the case of sawtooth type phase type elements, the diffraction efficiency is up to 100%. It can increase. The height of the tooth of the phase diffraction element is given by the following equation.

Where h _max is the maximum height of the tooth, m is the diffraction order, and n is the refractive index of the medium used.

The refractive index of the diffraction element for any wavelength is given by the following equation.

If the phase diffraction element has a kinoform, the diffraction efficiency is determined by the number of steps. In other words, if the number of steps is 4, 8, 16, the diffraction efficiency is 81%, 95%, 99%, respectively, which can be processed by lithography or diamond turning machine.

If the pitch of the diffraction element is properly designed, the diffraction element can be regarded as an equivalent aspherical lens because it can act as an aspherical lens. However, the diffraction element has a very large color dispersion characteristic unlike a conventional refractive lens.

Dispersion of Normal Refractive Lens (Abbe Number):

Dispersion of Diffractive Elements (Sweatt Model or High Refractive Index Method):

Where λ _d is the design center wavelength, λ _s is the short wavelength, and λ _l is the long wavelength.

Dispersion (v _dif ) and partial dispersion (P _{g, F} ) in the visible range:

The dispersion v _ref of a conventional refractive lens is 20 to 95 and the partial dispersion P _{g and F} have a value of 0.53 to 0.63. In contrast, diffractive lenses have a very large negative dispersion value. Therefore, the chromatic aberration can be corrected if the optical surface is composed of a mixed surface of the refractive surface and the diffractive surface.

When the diffractive surface and the refractive surface are simultaneously configured on the same surface, the total refractive power φ _tot is given as follows.

Here, φ _ref is the refractive power of the refraction element, and φ _dif is the refractive power of the diffraction element.

And the refractive power of each side to satisfy the molar coloration condition is given as follows.

In the plano-convex lens using diffraction and refraction surfaces, the chromatic aberration and magnification chromatic aberration can be corrected, but coma, image curvature, and distortion aberration cannot be sufficiently corrected. That is, even with a diffractive surface, high optical performance cannot be obtained with the flat-convex lens. Therefore, in order to obtain high optical performance with a single sheet, US Patent No. It is advisable to choose a meniscus shape that is concave towards the aperture, as shown in 5,949,577. In this patent, the curvature of two surfaces has a value smaller than the curvature of one surface, and the main beam incident on the axis is sufficiently refracted toward the upper surface to realize a wide angle of view. The diffractive surface is adopted on the second side than the first side to compensate for the chromatic aberration with relatively small refractive power, thereby reducing the secondary spectrum, which is the longitudinal chromatic aberration, relatively.

In a symmetric optical system with a magnification of 1, the latter half of the aperture (including the upper surface) becomes a mirror image of the first half and vice versa. In such a symmetrical optical system, the coma, distortion, lateral chromatic aberration and later aberration generated by the front half of the aperture have the same sign as the aberration, so these aberrations are zero. However, in general, it is difficult to configure the magnification to 1 in an optical system for image forming. Therefore, if the optical system is configured to have mutually symmetrical planes based on the aperture but not symmetrical, coma, distortion, and lateral chromatic aberration can be minimized.

1. Schematic diagram of a diffractive optical element

Figure 2. Lens schematic diagram from Example 1 to Example 9 of the present invention

3 is a lens configuration of Example 1 of the present invention.

4. Aberration for Example 1 of the Invention

5. MTF characteristics for Example 1 of the present invention

6. Magnification chromatic aberration for Example 1 of the present invention.

7 is a lens configuration of Example 2 of the present invention;

8. Aberration for Example 2 of the Invention

9. MTF characteristics for example 2 of the present invention

10. Magnification chromatic aberration for Example 2 of the present invention.

11 is a lens configuration of Example 3 of the present invention

12. Aberration for Example 3 of the Invention

13. MTF characteristics for example 3 of the present invention

14. Magnification chromatic aberration for Example 3 of the present invention.

15. A lens configuration of Example 10 of the present invention.

Figure 16. Aberration for Example 10 of the Invention

17. MTF Characteristics for Example 10 of the Invention

18. Magnification chromatic aberration for Example 10 of the present invention.

The present invention comprises a mixed lens using a diffractive surface and a refractive surface. (See Figs. 1 and 2)

The shape of the refractive and diffractive surface used in the present invention is as follows.

Optical axis symmetry aspherical equation (refractive surface):

C: center curvature, K: conic surface modulus, and A, B, C, D: aspherical modulus.

Phase equation of diffraction wavefront (diffractive surface):

Where λ ₀ is the design center wavelength, C ₁ (∝-1 / f), C ₂ (∝1 / f ³ ), C ₃ (∝-1 / f ⁵ ) is the phase coefficient, and m is the diffraction order (m Is mainly +1 order).

The refractive power of the diffractive element is given by the following equation.

The radius of the zone is such that the optical path difference differs by an integer multiple (k) of the design center wavelength (λ ₀ ) and is determined using the following equation.

The height of the teeth of the diffraction element is given by the formula (c), and the maximum number of zones is as follows.

Table 1 shows possible surface arrangements when the diffractive surface and the refractive surface (aspherical surface, spherical surface) are used in the lens design (see FIGS. 2 and 3). In addition, examples 1 to 10 are design values according to the various face arrangements shown in Table 1.

1) Design the lens with structurally small lateral color, coma, and distortion by constructing the same lens with mutual symmetry based on the aperture. Such a structure produces almost the same amount of surface deterioration based on the aperture even if surface deterioration (particularly, a relief structure of the diffractive surface) occurs when manufactured by plastic or glass molding, so the above-described aberrations are caused by mutual offsetting effects. Minimum.

2) It is composed of two diffraction and refraction type lenses, but since the same lens is used based on the aperture, the mold investment cost can be reduced when manufacturing with plastic and glass molding method.

3) Mixed lens with refractive and diffractive surface can correct various aberrations including chromatic aberration at high speed and wide angle of view compared to the pure lens. It can also reduce buying. This makes it possible to design a lens system for low cost, simple construction, and light and small image imaging.

4) When the diffraction surface is arranged on the diaphragm side, the maximum number of zones (diffraction rings) can be reduced, thereby minimizing the deterioration in optical performance by the diffractive optical element.

Claims (3)

In the mixed image forming lens using the diffractive surface and the refraction surface, the lens is composed of a total of two identical lenses having mutually symmetrical surfaces based on the aperture, and the shape of each surface is convex, concave, concave, convex or convex, convex, convex. And convex. In the lens system described above, the surface on which the diffractive surface and the refraction surface are used at the same time satisfies the following refractive power ratio. Where φ _dif is the refractive power of the refractive surface and φ _ref is the refractive power of the diffractive surface. In the above lens system, the focal length (f front _half or f _{rear half} ) and the concave surface (r ₂ or r ₄ ) of the front half lens or the rear half lens. The radius of curvature of satisfies the following conditions.